This invention relates to an estimation method for color reproduction characteristics using color patches suited for exactly estimating an input color component image signal corresponding to provided color coordinates, which reduces the color reproduction characteristics estimation processing time and improves the color reproduction characteristics.
For full color printers such as the general printers, thermal dye transfer printers, ink jet printers, and electrophotographic printers, four colors such as Yellow, Magenta, Cyan and Black (Y, M, C, and K) are often used as output colors of a color printer.
To represent all colors by using the four colors, there are various methods developed to combine Y, M, C, and K. Those methods are generally called "Under Color Removal" (UCR). The 100% UCR method is one of the concrete methods.
The 100% UCR method is a method to replace the smallest density of the original colors Y, M, and C (Y.sub.o, M.sub.o, and C.sub.o) with the density of the black K as shown in FIG. 24. When Y.sub.o is the smallest density as shown in the FIG. 24A, it is replaced as shown in the FIG. 24B by the 100% UCR method. The densities after replacement are Y.sub.n, M.sub.n, C.sub.n, and K.sub.n (Y.sub.n =0).
The 100% UCR method is characterized in that:
The operation reliability is increased.
The gray balance is stable.
Ink can be economized on.
Slight instability of ink can be eliminated.
The energy for drying ink can be conserved, and problems on drying are reduced in number. This is because the amount of K ink can be reduced to about 1/3 of the total amount of corresponding Y, M, and C inks.
To determine color reproduction characteristics of Y, M, C, and K, it is possible to prepare a few color patches using discrete combinations of Y, M, C, and K and to measure the patches by a colorimeter, and to estimate color reproduction characteristics of Y, M, C, and K of combinations other than the above by interpolating and operating the color values or to estimate a color value of some combination of Y, M, C, and K.
When specific color coordinates are specified inversely, the combination of Y, M, C, and K indicating the color coordinates can be estimated by interpolating and operating the actually measured color value using a color patch.
By estimating all combinations of Y, M, C, and K using color patches prepared on the basis of discrete combinations of Y, M, C, and K, the combination of Y, M, C, and K can be estimated from the actually measured color value. This method increases the estimation accuracy and improves the color reproduction characteristics.
FIG. 25 shows an example of color patches prepared by a discrete combination of Y, M, C, and K. In the figure, the maximum values of Y, M, C, and K are 2 to the 8th power, and the color patches provide color densities of five stages of 64 steps.
When color coordinates are provided as shown above or an electric signal (for example, R, G, and B color component image signals) equivalent to the color coordinates is provided, a color output correction means for obtaining a combination of Y, M, C, and K from the data is required.
FIG. 26 shows an example of the means. A color masking device 10, which is one of the color resolution image correction devices, is installed prior to a printer 12. The R, G, and B color resolution image signals equivalent to the color coordinates are converted to a combination of Y, M, C, and K color signals by the color masking device 10, which are supplied to the printer 12 to generate an image. The image is stored on a storage medium 13.
The 100% UCR method has the advantages mentioned above, though the method has the disadvantages given below, which may cause degradation of the color reproduction characteristics.
The saturation is generally lowered.
When the black component is increased, the density is decreased.
The present invention provides a method for improving the color reproduction characteristics, whose degradation is one of the disadvantages of the 100% UCR method, and for estimating color reproduction characteristics by using a fewer color patches when the color reproduction characteristics estimation method using color patches are to be adopted.